Short answer is that there are ways to make neutrons produce more than one triton. Neutrons can react with lithium-7 to produce tritium + a neutron (which can then go on to react with another lithium-6 or lithium-7 nucleus to produce more tritium). Neutron multipliers such as beryllium that react with neutrons to produce two neutrons can also be included in the tritium bleeding blanket.

Important to put the numbers in to context. They mention an increase from 0.5% land use for energy production to 3% in their net-zero scenario. It's a significant enough number that well thought out policy is needed, but according to this, 1.7% of the world's land is urban, 7.1% village, 14% cropland, and 26% pasture land. The 3% for energy production is a large increase in magnitude relative to existing energy infrastructure, but a drop in the bucket compared to total human land use (actually much less than I expected). Now add on top of this the fact that a lot the land for renewables will probably be dual use, such as wind farms on cropland, or solar on rooftops or pasture land, and it seems like it's actually quite a manageable number.